There has been a need for high performance reinforced plastic composites, such composites having a unique combination of reinforcement materials incorporated into a virgin plastic material, and such composites further having enhanced performance capabilities from a standpoint of durability and longevity, when exposed to wear mechanisms encountered in a typical tribological environment.
An engine driveline is one example of a tribological environment. In an engine driveline, and particularly in the design of a final drive in such a driveline, the use of plastic components for dynamic sealing and bearing applications is well known. In this environment i.e., where a sealing or bearing interface is involved, the plastic component is exposed to friction, pressure, high temperature and lubricants. One example of such a dynamic plastic component is a thrust washer, which is constantly subjected to a combination of varying levels of speed and load at high temperatures. Typically, a multiple of pressure and relative velocity (P*V) is a measure of how rigorous and demanding the application is. For example, a P*V value can range from as low as 50,000 to as high as 1,250,000, the pressure being measured in pounds/inch.sup.2 (psi) and the velocity being measured in feet/minute (fpm). Applications having a P*V greater than 150,000 are generally considered to be very demanding.
When a thrust washer is used in a dynamic sealing and bearing application, it eventually fails either due to excessive wear at a given P*V, or high thermal stresses due to poor heat dissipation i.e., "hot spotting", or sometimes a combination of both. Thus it is very desirable that the thrust washer have high wear resistance at a given P*V so that it performs as a bearing, that it have good heat dissipation properties to withstand thermal stresses and also, that it have good flexibility to provide toughness and perform as a seal. The thrust washers are typically made from plastics such as polyethersulphone (PES), polyamides (PA), polyetherketone (PEK) and polyphenylenesulphides (PPS), to name a few. These plastics are usually reinforced either with glass fibers, such as 40% glass filled PPS and in some cases, with carbon fibers.
It is well known in the art that the addition of one type of a reinforcing fiber in a plastic material increases the wear resistance of the plastic composite but consequently also increases the stiffness of the plastic composite. Because plastic parts such as thrust washers are generally injection molded, weld line strength of the molded part is also very significant. An increase in the wear resistance of an injection molded article made from a fiber reinforced plastic composite, is usually accompanied by a significant decrease in the weld line strength of that article, as compared to the virgin, or unfilled plastic. Poor weld line strength results in premature failure of a dynamic plastic component such as a thrust washer, when used in a demanding application.
U.S. Pat. No. 4,908,402 issued to Sasaki et al. on Mar. 13, 1990, describes a resin composition comprising polyglutarimide resin and carbon fibers incorporated therein. Although Sasaki et al. describe a plastic composite with carbon fibers having good heat resistance, wear resistance and thermal conductivity, they do not contemplate a wear additive package which is a unique combination of a plurality of fibers that preferably optimizes the desired properties such as high wear resistance and high weld line strength at a lower flexural modulus.
Besides thrust washers, there are other engine parts that are also exposed to tribological wear mechanisms. Piston rings made from plastic composites are constantly subjected to a harsh environment due to elevated temperatures encountered in the combustion chamber, as well as frictional wear and lubricants. Pump seals made from plastic composites perform the dual function of a seal and a bearing and thus require a combination of high wear resistance and weld line strength without a significantly high flexural modulus.
It is desirable to have a reinforced plastic composition that has excellent wear resistance and weld line strength properties without a significant increase in the flexural modulus. It is further desirable to have a fiber additive package that may be incorporated into different types of plastics to result in composite that has a combination of the above properties at a reasonable cost.
Heretofore, there has been no known plastic composition reinforced with a fiber additive package that results in a unique combination of the above desirable properties. There has also been no known plastic composition that exploits the heretofore unknown synergism between a combination of carbon and ceramic fibers, and ceramic and glass fibers, in a manner that results in an optimum combination of the above desired properties. The present invention is directed to overcome one or more of the problems as set forth above.